Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a multiplexer, such as a duplexer, for example, connected between an antenna (or multiple antennas as in the case of multiple input, multiple output (MIMO) designs) and a transceiver for filtering received and transmitted signals, typically within a predetermined radio frequency band. Other types of multiplexers in which the filters may be included are diplexers, triplexers, quadplexers, quintplexers and the like, for example. The multiplexer interfaces between the antenna and each of various networks to enable transmitting signals on different transmit (uplink) frequencies and receiving signals on different receive (downlink) frequencies. The filters associated with the multiplexer typically include band pass filters, which provide passbands for passing various transmitted and received signals through relatively narrow frequency bands (blocking all signals with frequencies outside the passbands).
Various types of filters use mechanical or acoustic resonators, such as surface acoustic wave (SAW) resonators. The mechanical/acoustic resonators convert electrical signals to mechanical signals or vibrations, and/or convert mechanical signals or vibrations to electrical signals. While certain surface modes are desired, certain standing spurious modes can exist between the opposing faces of the piezoelectric material of a SAW resonator. These spurious modes are parasitic, and can impact the performance of filters comprising SAW resonators.
In addition, many modern electronic devices communicate wirelessly more than one predetermined frequency or frequency band. For example, a wireless telephone may be designed to enable communications over two networks using radio frequency (RF) signals, such as Long-Term Evolution (LTE®) band B12 (uplink frequency band of 699 MHz-716 MHz and downlink frequency band of 729 MHz 746 MHz), and LTE band B26 (uplink frequency band of 814 MHz 849 MHz and downlink frequency band of 859 MHz-894 MHz), for example, although other types of networks and/or numbers of communication bands may be incorporated. Accordingly, this wireless telephone would require two transceivers and associated duplexers (including filters), one duplexer that enables transmitting and receiving band B12 signals and another for transmitting and receiving band B26 signals over one or more antennas.
Duplexer and higher band count multiplexers utilize single filter dies (for transmit and receive circuitry) for corresponding frequency bands, respectively. For example, in the above example, the wireless telephone would have one die for the band B12 duplexer and another die for the band B26 duplexer when using SAW resonator filters. (Generally, for filters comprising SAW resonators, there is one die for a duplexer, although for filters comprising thin-film bulk acoustic resonators (FBARs), there is typically one die per filter, not per duplexer.) Each die also includes its own antenna port for a dedicated or common antenna. Distribution on separate dies is necessitated, in part, by conventional die substrates, which are structurally inadequate to support the amount and density of circuitry (e.g., numerous acoustic resonators and band pass filters comprised of the same) required for operation of the wireless telephone over the multiple frequency bands. The multiple dies for the multiple frequency bands increase various costs, such as material costs, wafer costs, and die sorting costs. They also require additional space, which is at a premium when attempting to minimize the physical size of an electronic device.
As another example, for a conventional Quadplexer Integrated into a Single Chip (QISC), four dies are required (two transmit and two receive). For thin-film bulk acoustic resonator (FBAR) filters, each die is from a different wafer, where each wafer can have its own mask set (from 15 to 25 masks). Furthermore, each filter requires its own test, and if found “good” (passing), is placed in a tape and reel (T&R). But, if the die requires multiple RF tests prior to assembly, the die sort cost goes up. For SAW resonator devices, there is a reluctance to integrate multiple filter functions into a single die due to the fact that, as the die gets larger, the mechanical stresses on the SAW substrate are such that the part is likely to crack upon assembly, as mentioned above. Typically, SAW resonator devices are either single filter function or at most dual filter function (receive and transmit to form a duplexer).
Also, each acoustic resonator filter requires a wafer scale package. When doing four independent filters, for example, one can imagine four wafer scale packages for the four dies from the four wafers. Current wireless devices, such as iPhones, require many frequency bands to be covered, now more than 20 frequency bands. The filters, as well as a necessary “keep out” region around each filter, cause the filter area to be quite large. The “keep-out” area is due to the matching circuits buried under the die and assembly rules about minimum spacing for printed circuit board (PCB) loading.
What is needed, therefore, is a die capable of supporting multiple acoustic resonator structures for multiple frequency bands, overcoming at least the shortcomings of known SAW resonators described above.